Method and apparatus for spinal stabilization

ABSTRACT

A method of limiting at least one degree of movement between a superior vertebral body and an inferior vertebral body of a patient includes advancing a distal end of a stabilization device into a pedicle of the inferior vertebral body. A proximal portion of the stabilization device is positioned such that the proximal portion limits at least one degree of movement between a superior vertebral body and an inferior vertebral body by contacting a surface of the superior vertebral body.

PRIORITY INFORMATION

This application is a divisional of U.S. patent application Ser. No.11/056,991, filed Feb. 11, 2005, now U.S. Pat. No. 7,648,523 whichclaims the priority benefit under 35 U.S.C. §119(e) of ProvisionalApplication 60/634,203 filed Dec. 8, 2004, the contents of theseapplications are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to medical devices and, more particularly,to methods and apparatuses for spinal stabilization.

2. Description of the Related Art

The human spine is a flexible weight bearing column formed from aplurality of bones called vertebrae. There are thirty three vertebrae,which can be grouped into one of five regions (cervical, thoracic,lumbar, sacral, and coccygeal). Moving down the spine, there aregenerally seven cervical vertebra, twelve thoracic vertebra, five lumbarvertebra, five sacral vertebra, and four coccygeal vertebra. Thevertebra of the cervical, thoracic, and lumbar regions of the spine aretypically separate throughout the life of an individual. In contrast,the vertebra of the sacral and coccygeal regions in an adult are fusedto form two bones, the five sacral vertebra which form the sacrum andthe four coccygeal vertebra which form the coccyx.

In general, each vertebra contains an anterior, solid segment or bodyand a posterior segment or arch. The arch is generally formed of twopedicles and two laminae, supporting seven processes—four articular, twotransverse, and one spinous. There are exceptions to these generalcharacteristics of a vertebra. For example, the first cervical vertebra(atlas vertebra) has neither a body nor spinous process. In addition,the second cervical vertebra (axis vertebra) has an odontoid process,which is a strong, prominent process, shaped like a tooth, risingperpendicularly from the upper surface of the body of the axis vertebra.Further details regarding the construction of the spine may be found insuch common references as Gray's Anatomy, Crown Publishers, Inc., 1977,pp. 33-54, which is herein incorporated by reference.

The human vertebrae and associated connective elements are subjected toa variety of diseases and conditions which cause pain and disability.Among these diseases and conditions are spondylosis, spondylolisthesis,vertebral instability, spinal stenosis and degenerated, herniated, ordegenerated and herniated intervertebral discs. Additionally, thevertebrae and associated connective elements are subject to injuries,including fractures and torn ligaments and surgical manipulations,including laminectomies.

The pain and disability related to the diseases and conditions oftenresult from the displacement of all or part of a vertebra from theremainder of the vertebral column. Over the past two decades, a varietyof methods have been developed to restore the displaced vertebra totheir normal position and to fix them within the vertebral column.Spinal fusion is one such method. In spinal fusion, one or more of thevertebra of the spine are united together (“fused”) so that motion nolonger occurs between them. The vertebra may be united with varioustypes of fixation systems. These fixation systems may include a varietyof longitudinal elements such as rods or plates that span two or morevertebrae and are affixed to the vertebrae by various fixation elementssuch as wires, staples, and screws (often inserted through the pediclesof the vertebrae). These systems may be affixed to either the posterioror the anterior side of the spine. In other applications, one or morebone screws may be inserted through adjacent vertebrae to providestabilization.

Although spinal fusion is a highly documented and proven form oftreatment in many patients, there is currently a great interest insurgical techniques that provide stabilization of the spine whileallowing for some degree of movement. In this manner, the natural motionof the spine can be preserved, especially for those patients with mildor moderate disc conditions. In certain of these techniques, flexiblematerials are used as fixation rods to stabilize the spine whilepermitting a limited degree of movement.

Notwithstanding the variety of efforts in the prior art described above,these techniques are associated with a variety of disadvantages. Inparticular, these techniques typically involve an open surgicalprocedure, which results higher cost, lengthy in-patient hospital staysand the pain associated with open procedures.

Therefore, there remains a need for improved techniques and systems forstabilization the spine. Preferably, the devices are implantable througha minimally invasive procedure.

SUMMARY OF THE INVENTION

Accordingly, one embodiment of the present invention comprises a methodof limiting at least one degree of movement between a superior vertebralbody and an inferior vertebral body of a patient. A distal end of astabilization device is advanced into a pedicle of the inferiorvertebral body. A proximal portion of the stabilization device ispositioned such that the proximal portion limits at least one degree ofmovement between a superior vertebral body and an inferior vertebralbody by contacting a surface of the superior vertebral body.

Another embodiment of the present invention also comprises a method oflimiting at least one degree of movement between a superior vertebralbody and an inferior vertebral body of a patient. A distal end of afirst stabilization device is advanced into a pedicle of the inferiorvertebral body. A proximal portion of the first stabilization device ispositioned such that the proximal portion abuts against a surface of aninferior articular process of the superior adjacent vertebral body tolimit at least one degree of movement between a superior vertebral bodyand an inferior vertebral body. A distal end of a second stabilizationdevice is advanced into a pedicle of the inferior vertebral body suchthat it is positioned with bilateral symmetry with respect to the firststabilization device. A proximal portion of the second stabilizationdevice is positioned such that the proximal portion abuts, withbilateral symmetry with respect to the first stabilization device,against a surface of a second inferior articular process of the superioradjacent vertebral body to limit at least one degree of movement betweenthe superior vertebral body and the inferior vertebral body.

Another embodiment of the present invention comprises a spinalstabilization device that includes an elongate body, having a proximalend and a distal end. A distal anchor is on the distal end of theelongate body. A retention structure is on the body, proximal to thedistal anchor. A proximal anchor is moveably carried by the body. Theproximal anchor has an outer surface with at least a portion of theouter surface being elastic. At least one complementary retentionstructure on the proximal anchor configured for permitting proximalmovement of the body with respect to the proximal anchor but resistingdistal movement of the body with respect the proximal anchor.

Another embodiment of the present invention comprises a spinalstabilization device for limiting at least one degree of movementbetween a superior vertebral body and an inferior vertebral body of apatient. The device includes an elongate body that has a proximal endand a distal end. A distal anchor is positioned on the distal end of theelongate body. A retention structure is on the body, proximal to thedistal anchor. A proximal anchor is moveably carried by the body. Theproximal anchor includes at least one flat surface configured to abutagainst a surface of the inferior articular process of the superioradjacent vertebral body when the stabilization device is inserted intothe inferior adjacent vertebral body. At least one complementaryretention structure is on the proximal anchor and is configured forpermitting proximal movement of the body with respect to the proximalanchor but resisting distal movement of the body with respect theproximal anchor.

Yet another embodiment of the present invention comprises a spinalstabilization device for limiting at least one degree of movementbetween a superior vertebral body and an inferior vertebral body of apatient. The device comprises an elongate body, having a proximal endand a distal end. A distal anchor is on the distal end of the elongatebody. A retention structure is positioned on the body, proximal to thedistal anchor. A proximal anchor is moveably carried by the body. Theproximal anchor includes at least one saddle-shaped surface configuredto abut against an inferior articular process of the superior adjacentvertebral body when the stabilization device is inserted into theinferior adjacent vertebral body. At least one complementary retentionstructure is on the proximal anchor and is configured for permittingproximal movement of the body with respect to the proximal anchor butresisting distal movement of the body with respect the proximal anchor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A a side elevational view of a portion of a vertebra having anexemplary embodiment of a stabilization device implanted therein.

FIG. 1B is a posterior view of a portion of a vertebra having twodevices similar to that of FIG. 1A implanted bilaterally therein.

FIG. 2 is a side perspective view of the stabilization device of FIGS.1A and 1B.

FIG. 3 is a side view of the stabilization device of FIG. 2.

FIG. 3A is a cross-sectional view of a body portion of the stabilizationdevice of FIG. 2.

FIG. 4 is a partial cross-sectional view of a proximal portion of thestabilization device of FIG. 2.

FIG. 5 is an enlarged view of a portion of FIG. 4 labeled 5-5.

FIG. 6 is a side perspective view of a locking ring of the stabilizationdevice of FIG. 3.

FIG. 7 is a side view of a modified embodiment of a body portion of thestabilization device shown in FIG. 2.

FIG. 7A is an enlarged view of a portion of FIG. 7 labeled 7A-7A.

FIG. 8 is a side view of another modified embodiment of the proximalanchor.

FIG. 9 is a top perspective view of a modified embodiment of theproximal anchor.

FIG. 9A is a side view of another modified embodiment of the proximalanchor.

FIG. 10 is a side view of another modified embodiment of the proximalanchor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the stabilization devices of the present invention will bedisclosed primarily in the context of a spinal stabilization procedure,the methods and structures disclosed herein are intended for applicationin any of a variety medical applications, as will be apparent to thoseof skill in the art in view of the disclosure herein. For example,certain features and aspects of bone stabilization device and techniquesdescribed herein may be applicable to proximal fractures of the femurand a wide variety of fractures and osteotomies, the hand, such asinterphalangeal and metacarpophalangeal arthrodesis, transversephalangeal and metacarpal fracture fixation, spiral phalangeal andmetacarpal fracture fixation, oblique phalangeal and metacarpal fracturefixation, intercondylar phalangeal and metacarpal fracture fixation,phalangeal and metacarpal osteotomy fixation as well as others known inthe art. See e.g., U.S. Pat. No. 6,511,481, which is hereby incorporatedby reference herein. A wide variety of phalangeal and metatarsalosteotomies and fractures of the foot may also be stabilized using thebone fixation devices described herein. These include, among others,distal metaphyseal osteotomies such as those described by Austin andReverdin-Laird, base wedge osteotomies, oblique diaphyseal, digitalarthrodesis as well as a wide variety of others that will be known tothose of skill in the art. Fractures of the fibular and tibial malleoli,pilon fractures and other fractures of the bones of the leg may befixated and stabilized with these bone fixation devices with or withoutthe use of plates, both absorbable or non-absorbing types, and withalternate embodiments of the current invention The stabilization devicesmay also be used to attach tissue or structure to the bone, such as inligament reattachment and other soft tissue attachment procedures.Plates and washers, with or without tissue spikes for soft tissueattachment, and other implants may also be attached to bone, usingeither resorbable or nonresorbable fixation devices depending upon theimplant and procedure. The stabilization devices may also be used toattach sutures to the bone, such as in any of a variety of tissuesuspension procedures. The bone stabilization device described hereinmay be used with or without plate(s) or washer(s), all of which can beeither permanent, absorbable, or combinations.

FIGS. 1A and 1B are side and rear elevational views of a pair of bonestabilization devices 12, positioned within a vertebra 10 of the spine.As will be explained in more detail below, the bone stabilizationdevices 12 may be used in a variety of techniques to stabilize thespine. For example, in the illustrated embodiment, the distal end of thebone stabilization device 12 is inserted into the pedicle of thevertebrae, preferably through the pars (i.e., the region of the laminabetween the superior and inferior articular processes). The proximal endof the device 12 extends above the pars such that it limits motion ofthe superior adjacent vertebrae with respect to the inferior articularprocess. In one embodiment, the proximal end of the device limits motionby abutting and/or wedging against a surface of the superior adjacentvertebrae as the superior adjacent vertebrae moves relative to theinferior adjacent vertebrae. In this manner, at least one degree ofmotion between the inferior and superior vertebrae may be limited. Asexplained below, the bone stabilization devices 12 may be used afterlaminectomy, discectomy, artificial disc replacement, and otherapplications for providing temporary or permanent stability in thespinal column. For example, lateral or central spinal stenosis may betreated with the bone fixation devices 12 and techniques describedbelow. In such procedures, the bone fixation devices 12 and techniquesmay be used alone or in combination with laminectomy, discectomy,artificial disc replacement, and/or other applications for relievingpain and/or providing stability.

An embodiment of the stabilization device 12 will now be described indetail with initial reference to FIGS. 2-4. The stabilization device 12comprises a body 28 that extends between a proximal end 30 and a distalend 32. The length, diameter and construction materials of the body 28can be varied, depending upon the intended clinical application. Inembodiments optimized for spinal stabilization in an adult humanpopulation, the body 28 will generally be within the range of from about20-90 mm in length and within the range of from about 3.0-8.5 mm inmaximum diameter. The length of the helical anchor, discussed below, maybe about 8-80 millimeters. Of course, it is understood that thesedimensions are illustrative and that they may be varied as required fora particular patient or procedure.

In one embodiment, the body 28 comprises titanium. However, as will bedescribed in more detail below, other metals, or bioabsorbable ornonabsorbable polymeric materials may be utilized, depending upon thedimensions and desired structural integrity of the finishedstabilization device 12.

The distal end 32 of the body 28 is provided with a cancellous boneanchor or distal cortical bone anchor 34. Generally, for spinalstabilization, the distal bone anchor 34 is adapted to be rotationallyinserted into a portion (e.g., the facet or pedicle) of a firstvertebra. In the illustrated embodiment, the distal anchor 34 comprisesa helical locking structure 72 for engaging cancellous and/or distalcortical bone. In the illustrated embodiment, the locking structure 72comprises a flange that is wrapped around a central core 73, which inthe illustrated embodiment is generally cylindrical in shape. The flange72 extends through at least one and generally from about two to about 50or more full revolutions depending upon the axial length of the distalanchor 34 and intended application. The flange will generally completefrom about 2 to about 20 revolutions. The helical flange 72 ispreferably provided with a pitch and an axial spacing to optimize theretention force within cancellous bone.

The helical flange 72 of the illustrated embodiment has a generallytriangular cross-sectional shape (see FIG. 3). However, it should beappreciated that the helical flange 72 can have any of a variety ofcross sectional shapes, such as rectangular, oval or other as deemeddesirable for a particular application through routine experimentationin view of the disclosure herein. For example, in one modifiedembodiment, the flange 72 has a triangular cross-sectional shape with ablunted or square apex. The outer edge of the helical flange 72 definesan outer boundary. The ratio of the diameter of the outer boundary tothe diameter of the central core 73 can be optimized with respect to thedesired retention force within the cancellous bone and giving dueconsideration to the structural integrity and strength of the distalanchor 34. Another aspect of the distal anchor 34 that can be optimizedis the shape of the outer boundary and the central core 73, which in theillustrated embodiment are generally cylindrical.

The distal end 32 and/or the outer edges of the helical flange 72 may beatraumatic (e.g., blunt or soft). This inhibits the tendency of thestabilization device 12 to migrate anatomically distally and potentiallyout of the vertebrae after implantation. Distal migration is alsoinhibited by the dimensions and presence of a proximal anchor 50, whichwill be described below. In the spinal column, distal migration isparticularly disadvantageous because the distal anchor 34 may harm thetissue, nerves, blood vessels and/or spinal cord which lie within and/orsurround the spine. In other embodiments, the distal end 32 and/or theouter edges of the helical flange 72 may be sharp and/or configured suchthat the distal anchor 34 is self tapping and/or self drilling.

A variety of other embodiments for the distal anchor 32 can also beused. For example, the various distal anchors described in co-pendingU.S. patent application Ser. No. 10/012,687, filed Nov. 13, 2001 can beincorporated into the stabilization device 12 described herein. Theentire contents of this application are hereby expressly incorporated byreference. In particular, the distal anchor 32 may comprise a singlehelical thread surrounding a lumen, much as in a conventional corkscrew.Alternatively, a double helical thread may be utilized, with the distalend of the first thread rotationally offset from the distal end of thesecond thread. The use of a double helical thread can enable a greateraxial travel for a given degree of rotation and greater retention forcethan a corresponding single helical thread. Specific distal anchordesigns can be optimized for the intended use, taking into accountdesired performance characteristics, the integrity of the distal bone,and whether the distal anchor is intended to engage exclusivelycancellous bone or will also engage cortical bone. In still otherembodiments, the distal anchor 34 may be formed without a helicalflange. For example, various embodiments of levers, prongs, hooks and/orradially expandable devices may also be used. See e.g., U.S. Pat. No.6,648,890, which is hereby expressly incorporated by reference in itsentirety.

As shown in FIG. 3A, the body 28 is cannulated forming a central lumen42 to accommodate installation over a placement wire as is understood inthe art. The cross section of the illustrated central lumen is circularbut in other embodiments may be non circular, e.g., hexagonal, toaccommodate a corresponding male tool for installation or removal of thebody 28 as explained below. In other embodiments, the body 28 maypartially or wholly solid.

With continued reference to FIGS. 2-4, the proximal end 30 of the body28 is provided with a rotational coupling 70, for allowing the body 28to be rotated. Rotation of the rotational coupling 70 can be utilized torotationally drive the distal anchor 32 into the bone. In suchembodiments, any of a variety of rotation devices may be utilized, suchas electric drills or hand tools, which allow the clinician to manuallyrotate the proximal end 30 of the body 28. Thus, the rotational coupling70 may have any of a variety of cross sectional configurations, such asone or more curved faces, flats or splines. In the illustratedembodiment, the rotational coupling 70 is a male element in the form ofa hexagonal projection. However, in other embodiments, the rotationalcoupling 70 may be in the form of a female component, machined, milledor attached to the proximal end 30 of the body 28. For example, in onesuch embodiment, the rotational coupling 70 comprises an axial recesswith a polygonal cross section, such as a hexagonal cross section. Asexplained above, the axial recess may be provided as part of the centrallumen 42.

The proximal end 30 of the fixation device is also provided with aproximal anchor 50. The proximal anchor 50 comprises a housing 52, whichforms a lumen 53 (see FIG. 5) configured such that the body 28 mayextend, at least partially, through the proximal anchor 50. The proximalanchor 50 is axially distally moveable along the body 28, to permitcompression of between the distal and proximal ends 32, 30 of thestabilization device 12. As will be explained below, complimentarylocking structures such as threads, levers, split rings, and/or ratchetlike structures between the proximal anchor 50 and the body 28 resistproximal movement of the anchor 50 with respect to the body 28 undernormal use conditions. The proximal anchor 50 preferably can be axiallyadvanced along the body 28 with and/or without rotation as will beapparent from the disclosure herein.

With particular reference to FIGS. 4-6, in the illustrated embodiment,the complementary structure of the proximal anchor 50 is formed by anannular ring 51, which is positioned within an annular recess 55 formedalong the lumen 53. As will be explained below, the ring 51 comprisessurface structures 54 which interact with complimentary surfacestructures 58 on the body 28. In the illustrated embodiment, thecomplimentary surface structures 58 comprise a series of annular ridgesor grooves 60 formed on the surface of the body 28. The surfacestructures 54 and complementary surface structures 58 permit distalaxial travel of the proximal anchor 50 with respect to the body 28, butresist proximal travel of the proximal anchor 50 with respect to thebody 28 as explained below.

As shown in FIG. 6, the annular ring 51 is split (i.e., has a least onegap) and is interposed between the body 28 and the recess 55 of theproximal anchor 50 (see FIG. 5). In the illustrated embodiment, the ring51 comprises a tubular housing 57 (see FIG. 6), which defines a gap orspace 59. In one embodiment, the gap 59 is defined by a pair of edges 61a, 61 b, that are generally straight and parallel to each other.Although not illustrated, it should be appreciated that in modifiedembodiments, the ring 51 can be formed without a gap. When the ring 51is positioned along the body 28, the ring 51 preferably surrounds asubstantial portion of the body 28. The ring 51 can be configured sothat the ring 51 can flex or move radially outwardly in response to anaxial force so that the ring 51 can be moved relative to the body 28, asdescribed below.

In the illustrated embodiment, the tubular housing 57 includes at leastone and in the illustrated embodiment ten teeth or flanges 63, which areconfigured to engage the complementary surface structures 58 on the body28 in a ratchet-like motion. In the illustrated embodiment (see FIG. 5),the teeth or flanges include a first surface 65 that lies generallyperpendicular to the longitudinal axis of the anchor and generally facesthe proximal direction (i.e., the direction labeled “P” in FIG. 5) and asecond surface 67 that is inclined with respect to the longitudinal axisof the anchor and that faces distal direction (i.e., the directionlabeled “D” in FIG. 5). It should be noted that the proximal anddirections in FIG. 5 are reversed with respect to FIG. 4.

With continued reference to FIG. 5, the recess 55 is sized anddimensioned such that as the proximal anchor 50 is advanced distallyover the body, the second surface 67 of the annular ring 51 can slidealong and over the complementary retention structures 58 of the body 28.That is, the recess 55 provides a space for the annular ring to moveradially away from the body 28 as the proximal anchor 50 is advanceddistally.

A distal portion 69 of the recess 55 is sized and dimensioned such thatafter the proximal anchor 50 is appropriately advanced, proximal motionof the proximal anchor 50 is resisted as the annular ring 51 becomeswedged between the body 28 and an angled engagement surface 71 of thedistal portion 69. In this manner, proximal movement of the proximalanchor 50 under normal use conditions may be prevented. In modifiedembodiments, the annular ring 51 can be sized and dimensioned such thatthe ring 51 is biased inwardly to engage the retention structures 58 onthe body 28. The bias of the annular ring 51 can result in a moreeffective engagement between the complementary retention structures 58of the body and the retention structures 54 of the ring 51.

As mentioned above, it is contemplated that various other retentionstructures 54 and complementary retention structures 58 may be usedbetween the body 28 and the proximal anchor 50 to permit distal axialtravel of the proximal anchor 50 with respect to the body 28, but resistproximal travel of the proximal anchor 50 with respect to the body 28.Examples of such structures can be found in U.S. Pat. No. 6,685,706,entitled “PROXIMAL ANCHORS FOR BONE FIXATION SYSTEM.” The entirecontents of this patent is hereby expressly incorporated by referenceherein.

As mentioned above, the complimentary surface structures 58 on the body28 comprise a series of annular ridges or grooves 60. These retentionstructures 58 are spaced axially apart along the body 28, between aproximal limit 62 and a distal limit 64. See FIG. 4. The axial distancebetween proximal limit 62 and distal limit 64 is related to the desiredaxial working range of the proximal anchor 50, and thus the range offunctional sizes of the stabilization device 12. Thus, the stabilizationdevice 12 of the exemplary embodiment can provide compression betweenthe distal anchor 34 and the proximal anchor 50 throughout a range ofmotion following the placement of the distal anchor in a vertebra. Thatis, the distal anchor 34 may be positioned within the cancellous and/ordistal cortical bone of a vertebra, and the proximal anchor may bedistally advanced with respect to the distal anchor throughout a rangeto provide compression without needing to relocate the distal anchor 34and without needing to initially locate the distal anchor 34 in aprecise position with respect to the proximal side of the bone oranother vertebra. Providing a working range throughout which tensioningof the proximal anchor 50 is independent from setting the distal anchor34 allows a single device to be useful for a wide variety of differentanatomies, as well as eliminates or reduces the need for accurate devicemeasurement. In addition, this arrangement allows the clinician toadjust the compression force during the procedure without adjusting theposition of the distal anchor. In this manner, the clinician may focuson positioning the distal anchor sufficiently within the vertebra toavoid or reduce the potential for distal migration out of the vertebra,which may damage the particularly delicate tissue, blood vessels, nervesand/or spinal cord surrounding or within the spinal column. In additionor alternative, the above described arrangement allows the clinician toadjust the positioning of the proximal anchor 50 with respect to theinferior articular process of the superior adjacent vertebrae. In thismanner, the clinician may adjust the position of the proximal anchor 50without adjusting the position of the distal anchor such that the anchor50 is configured to wedge or abut against inferior articular process ofthe superior adjacent vertebrae.

In many applications, the working range is at least about 10% of theoverall length of the device, and may be as much as 20% or 50% or moreof the overall device length. In the context of a spinal application,working ranges of up to about 10 mm or more may be provided, sinceestimates within that range can normally be readily accomplished withinthe clinical setting. The embodiments disclosed herein can be scaled tohave a greater or a lesser working range, as will be apparent to thoseof skill in the art in view of the disclosure herein.

With reference back to FIGS. 2-4, in the illustrated embodiment, theouter surface 49 of the proximal anchor 50 has a smooth or sphericalshape. As will be explained below, the outer surface 49 of the proximalanchor 50 is configured to abut against the inferior facet of thesuperior adjacent vertebrae. In this manner, motion between the adjacentvertebrae may be limited and/or constrained.

FIG. 7 illustrates an embodiment in which the body 28 comprises a firstportion 36 and a second portion 38 that are coupled together at ajunction 40. In the illustrated embodiment, the first portion 36 carriesthe distal anchor 34 (shown without a central core) while the secondportion 38 forms the proximal end 30 of the body 28. As will beexplained in more detail below, in certain embodiments, the secondportion 38 may be used to pull the body 28 and therefore will sometimesbe referred to as a “pull-pin.” The first and second portions 36, 38 arepreferably detachably coupled to each other at the junction 40. In theillustrated embodiment, the first and second portions 36, 38 aredetachably coupled to each other via interlocking threads. Specifically,as best seen in FIG. 7A, the body 28 includes an inner surface 41, whichdefines a central lumen 42 that preferably extends from the proximal end30 to the distal end 32 throughout the body 28. At the proximal end ofthe first portion 36, the inner surface 41 includes a first threadedportion 44. The first threaded portion 44 is configured to mate with asecond threaded portion 46, which is located on the outer surface 45 ofthe second portion 38. The interlocking annular threads of the first andsecond threaded portions 44, 46 allow the first and second portions 36,38 to be detachably coupled to each other. In one modified embodiment,the orientation of the first and second threaded portions 44, 46 can bereversed. That is, the first threaded portion 44 can be located on theouter surface of the first portion 36 and the second threaded portion 46can be located on the inner surface 41 at the distal end of the secondportion 38. Any of a variety of other releasable complementaryengagement structures may also be used, to allow removal of secondportion 38 following implantation, as is discussed below.

In a modified arrangement, the second portion 38 can comprise any of avariety of tensioning elements for permitting proximal tension to beplaced on the distal anchor 34 while the proximal anchor is advanceddistally to compress the fracture. For example, any of a variety oftubes or wires can be removably attached to the first portion 36 andextend proximally to the proximal handpiece. In one such arrangement,the first portion 36 can include a releasable connector in the form of alatching element, such as an eye or hook. The second portion 38 caninclude a complementary releasable connector (e.g., a complementaryhook) for engaging the first portion 36. In this manner, the secondportion 38 can be detachably coupled to the first portion 36 suchproximal traction can be applied to the first portion 36 through thesecond portion as will be explained below. Alternatively, the secondportion 48 may be provided with an eye or hook, or transverse bar,around which or through which a suture or wire may be advanced, bothends of which are retained at the proximal end of the device. Followingproximal tension on the tensioning element during the compression step,one end of the suture or wire is released, and the other end may bepulled free of the device. Alternate releasable proximal tensioningstructures may be devised by those of skill in the art in view of thedisclosure herein.

In a final position, the distal end of the proximal anchor 50 preferablyextends distally past the junction 40 between the first portion 36 andthe second portion 38. As explained above, the proximal anchor 50 isprovided with one or more surface structures 54 for cooperating withcomplementary surface structures 58 on the first portion 36 of the body28.

In this embodiment, the stabilization device 12 may include anantirotation lock (not shown) between the first portion 36 of the body28 and the proximal collar 50. For example, the first portion 36 mayinclude one or more of flat sides (not shown), which interact withcorresponding flat structures in the proximal collar 50. As such,rotation of the proximal collar 50 is transmitted to the first portion36 and distal anchor 34 of the body 28. Of course, those of skill in theart will recognize various other types of splines or other interfitstructures can be used to prevent relative rotation of the proximalanchor and the first portion 36 of the body 28. To rotate the proximalanchor 50, the housing 52 may be provided with a gripping structure (notshown) to permit an insertion tool to rotate the flange proximal anchor50. Any of a variety of gripping structures may be provided, such as oneor more slots, recesses, protrusions, flats, bores or the like. In oneembodiment, the proximal end of the proximal anchor 50 is provided witha polygonal, and, in particular, a pentagonal or hexagonal recess orprotrusion.

With reference to FIG. 8, in a modified embodiment, the distal end ofthe proximal anchor 50 may include one or more bone engagement features100, which in the illustrated embodiment comprises a one or more spikes102 positioned on a contacting surface 104 of the proximal anchors. Thespikes 102 provide additional gripping support especially when theproximal anchor 50 is positioned against, for example, uneven bonesurfaces and/or soft tissue. In addition, the spikes 102 may limitrotation of the proximal anchor 50 with respect to the body 28 therebypreventing the proximal anchor 50 from backing off the body 28. Otherstructures for the bone engagement feature 100 may also be used, suchas, for example, ridges, serrations etc.

Methods implanting stabilization devices described above as part of aspinal stabilization procedure will now be described. Although certainaspects and features of the methods and instruments described herein canbe utilized in an open surgical procedure, the disclosed methods andinstruments are optimized in the context of a percutaneous or minimallyinvasive approach. Thus, the method steps which follow and thosedisclosed are intended for use in a trans tissue approach. However, tosimplify the illustrations, the soft tissue adjacent the treatment sitehave not been illustrated in the drawings.

In one embodiment of use, a patient with a spinal instability isidentified. The patient is preferably positioned face down on anoperating table, placing the spinal column into a flexed position. Atrocar may then be inserted through a tissue tract and advanced towardsa first vertebral body. A guidewire may then be advanced through thetrocar and into the first vertebral body. With reference to FIGS. 1A and1B, the guide wire is preferably inserted into the pedicle of thevertebral body preferably through the pars (i.e. the region of thelamina between the superior and inferior articular processes). Asuitable tissue expander may then be inserted over the guidewire andexpanded to enlarge the tissue tract. A surgical sheath may then beadvanced over the expanded tissue expander.

A drill with a rotatable tip may be advanced over the guidewire andthrough the sheath. The drill may be used to drill an opening in thevertebral body. The opening may be configured for (i) for insertion ofthe body 28 of the bone stabilization device 12, (ii) taping and/or(iii) providing a counter sink for the proximal anchor 50. In otherembodiments, the step of drilling may be omitted. In such embodiments,the distal anchor 34 is preferably self-tapping and self drilling.

The body 28 of the fixation device may be advanced over the guidewireand through the sheath until it engages the vertebral body. The body 28may be coupled to a suitable insertion tool prior to the step ofengaging the fixation device 12 with the vertebral body. The insertiontool may be configured to engage the coupling 70 on the proximal end ofthe body 28 such that insertion tool may be used to rotate the body 28.In such an embodiment, the fixation device 12 is preferably configuredsuch that it can also be advanced over the guidewire.

The insertion tool may be used to rotate the body 28 thereby driving thedistal anchor 34 to the desired depth within the pedicle of thevertebral body. The proximal anchor 50 may be carried by the fixationdevice prior to advancing the body 28 into the vertebrae, or may beattached following placement (partially or fully) of the body 28 withinthe vertebrae. In one embodiment, the clinician will have access to anarray of devices 12, having, for example, different diameters, axiallengths, configurations and/or shapes. The clinician will assess theposition of the body 28 with respect to the superior vertebral body andchose the proximal anchor 50 from the array, which best fits the patientanatomy to achieve the desired clinical result.

Once the distal anchor 34 is in the desired location, the proximalanchor 50 is preferably advanced over the body 28 until it reaches itsdesired position. This may be accomplished by pushing on the proximalanchor 50 or by applying a distal force to the proximal anchor 50. Inanother embodiment, the proximal anchor 50 is advanced by applying aproximal retraction force to the proximal end 30 of body 28, such as byconventional hemostats, pliers or a calibrated loading device, whiledistal force is applied to the proximal anchor 50. In this manner, theproximal anchor 50 is advanced distally with respect to the body 28until the proximal anchor 50 is in its proper position (e.g., positionedsnugly against the outer surface of the vertebra.) Appropriatetensioning of the stabilization device 12 can be accomplished by tactilefeedback or through the use of a calibration device for applying apredetermined load on the stabilization device 12. As explained above,one advantage of the structure of the illustrated embodiments is theability to adjust compression independently of the setting of the distalanchor 34 within the vertebra.

Following appropriate tensioning of the proximal anchor 50, the proximalportion of the body 28 extending proximally from the proximal anchor 50can be removed. In one embodiment, this may involve cutting the proximalend of the body 28. For example, the proximal end of the body may beseparated by a cutting instrument or by cauterizing. Cauterizing mayfuse the proximal anchor 50 to the body 32 thereby adding to theretention force between the proximal anchor 50 and the body 28. Suchfusion between the proximal anchor and the body may be particularlyadvantageous if the pin and the proximal anchor are made from apolymeric or plastic material. In this manner, as the material of theproximal anchor and/or the pin is absorbed or degrades, the fusioncaused by the cauterizing continues to provide retention force betweenthe proximal anchor and the body. In another embodiment, the bodycomprises a first and a second portion 36, 38 as described above. Insuch an embodiment, the second portion 38 may detached from the firstportion 36 and removed. In the illustrated embodiment, this involvesrotating the second portion 38 with respect to the first portion via thecoupling 70. In still other embodiments, the proximal end of the body 28may remain attached to the body 28.

The access site may be closed and dressed in accordance withconventional wound closure techniques and the steps described above maybe repeated on the other side of the vertebral body for bilateralsymmetry as shown in FIGS. 1A and 1B. The bone stabilization devices 12may be used alone or in combination laminectomy, discectomy, artificialdisc replacement, and/or other applications for relieving pain and/orproviding stability.

It should be appreciated that not all of the steps described above arecritical to procedure. Accordingly, some of the described steps may beomitted or performed in an order different from that disclosed. Further,additional steps may be contemplated by those skilled in the art in viewof the disclosure herein, without departing from the scope of thepresent invention.

With reference to FIGS. 1A and 1B, the proximal anchors 50 of thedevices 12 extend above the pars such that they abut against theinferior facet of the superior adjacent vertebrae. In this manner, theproximal anchor 50 forms a wedge between the vertebral bodies limitingcompression of the spine as the facet of the superior adjacent vertebraeabuts against the proximal anchor 50. In this manner, compression islimited while other motion is not. For example, flexion, lateralmovement and torsion between the superior and inferior vertebral bodiesis not limited or constrained. In this manner, the natural motion of thespine can be preserved, especially for those patients with mild ormoderate disc conditions. Preferably, the devices are implantablethrough a minimally invasive procedure and, more preferably, through theuse of small percutaneous openings as described above. In this manner,the high cost, lengthy in-patient hospital stays and the pain associatedwith open procedures can be avoided and/or reduced. In one embodiment,the devices 12 may be removed and/or proximal anchors 50 may be removedin a subsequent procedure if the patient's condition improves. Onceimplant, it should be appreciated that, depending upon the clinicalsituation, the proximal anchor 50 may be positioned such that itcontacts surfaces of the adjacent vertebrae all of the time, most of thetime or only when movement between the adjacent vertebrae exceeds alimit.

In the embodiments described above, it may be advantageous to allow theproximal anchor to rotate with respect to the body 28 thereby preventingthe proximal anchor 50 from causing the distal anchor 34 from backingout of the pedicle. In another embodiment, engagement features 100 maybe added to the proximal anchor 50 as described above to preventrotation of the proximal anchor 50.

The fixation devices 12 may be made from conventional non-absorbable,biocompatible materials including stainless steel, titanium, alloysthereof, polymers, composites and the like and equivalents thereof. Inone embodiment, the distal anchor comprises a metal helix, while thebody and the proximal anchor comprise a bioabsorbable material.Alternatively, the distal anchor comprises a bioabsorbable material, andthe body and proximal anchor comprise either a bioabsorbable material ora non-absorbable material.

In one embodiment, the proximal anchor 50 is formed, at least in part,from an elastic and/or resilient material. In this manner, the shock andforces that are generated as the proximal anchor abuts or wedges againstthe inferior articular process of the superior adjacent vertebrae can bereduced or dissipated. In one such embodiment, the proximal anchor 50 isformed in part by a polycarbonate urethane or a hydrogel. In suchembodiments, the elastic material may be positioned on the outersurfaces of the proximal anchor or the portions of the outer surfacesthat abut against the surfaces of the inferior articular process of thesuperior adjacent vertebrae.

For example, FIG. 9 illustrates an embodiment of a proximal anchor 50′,which comprises an outer housing or shell 202. The shell 202 may beformed or a resilient material such as, for example, a biocompatiblepolymer. The proximal anchor 50′ also comprises an inner member 204 thatcomprises a tubular housing 206 and a proximal flange 208. The innermember 202 is preferably formed of a harder more rugged material ascompared to the shell 202, such as, for example, titanium or anothermetallic material. The shell 202 is fitted or formed over the tubularhousing 206. When deployed, the shell 202 is held in place between theflange 208 and the surface of the vertebrae in which the body 202 isplaced. In modified embodiments, the shell 202 may be coupled to theinner member 204 in a variety of other manners, such as, adhesives,fasteners, interlocking surfaces structures and the like. In theillustrated embodiment, the inner member 204 includes a locking ring 51positioned within a recess 55 as described above. Of course, in modifiedembodiments, other retention structures 54 and complementary retentionstructures 58 may be used between the body 28 and the proximal anchor50′ to permit distal axial travel of the proximal anchor 50′ withrespect to the body 28, but resist proximal travel of the proximalanchor 50′ with respect to the body 28.

FIGS. 9A and 10 illustrate modified shapes of the proximal anchor whichcan be used alone or in combination with the elastic or resilientmaterial described above. In FIG. 9A, the proximal anchor 250 has asaddle shaped curved surface 251 that generally faces the inferiorarticular process of the superior adjacent vertebrae. In thisembodiment, the saddle shaped surface may limit compression of theadjacent vertebral bodies and limit side to side motion and/or torsionbetween the vertebral bodies. FIG. 10 illustrates an embodiment in whichthe proximal anchor 350 has a rectangular shape with a flat shapedsurface 351. In this embodiment, the flat shaped surface may limitcompression of the adjacent vertebral bodies and limit side to sidemotion between the vertebral bodies. In the embodiments of FIGS. 9A and10, in may be advantageous to limit or eliminate any rotation of theproximal anchor 50 with respect to the body 28 and/or the vertebralbody. As such, the proximal anchor 50 preferably includes the retentiondevices 100 described above with reference to FIG. 8.

The above described devices and techniques limit motion of the spine byproviding an abutment or wedge surface on one vertebral body. Theabutment surface contacts a portion of a second, adjacent vertebral bodyso as limit least one degree of motion between the two vertebral bodieswhile permitting at least one other degree of motion. While the abovedescribed devices and techniques are generally preferred, certainfeatures and aspects can be extended to modified embodiments forlimiting motion between vertebral bodies. These modified embodimentswill now be described.

In one embodiment, the proximal anchor 50 of the fixation device may be,coupled to attached or integrally formed with the body 28. In thismanner, movement between the proximal anchor 50 and the body 28 is notpermitted. Instead, the clinician may chose a fixation device of theproper length and advance the device into the vertebral body until theproximal anchor lies flush with the vertebral body or is otherwisepositioned accordingly with respect to the vertebral body.

In another embodiment, the abutment surface may be attached to thevertebral body through the use of an adhesive, fasteners, staples,screws and the like In still another embodiment, the abutment surfacemay formed on a distal end of a stabilization device that is insertedthrough the front side of the vertebral body.

Preferably, the clinician will have access to an array of fixationdevices 12, having, for example, different diameters, axial lengths and,if applicable, angular relationships. These may be packaged one or moreper package in sterile or non-sterile envelopes or peelable pouches, orin dispensing cartridges which may each hold a plurality of devices 12.The clinician will assess the dimensions and load requirements, andselect a fixation device from the array, which meets the desiredspecifications.

The fixation devices may also be made from conventional non-absorbable,biocompatible materials including stainless steel, titanium, alloysthereof, polymers, composites and the like and equivalents thereof. Inone embodiment, the distal anchor comprises a metal helix, while thebody and the proximal anchor comprise a bioabsorbable material. Inanother embodiment, the body is made of PEEK™ polymer or similar plasticmaterial. Alternatively, the distal anchor comprises a bioabsorbablematerial, and the body and proximal anchor comprise either abioabsorbable material or a non-absorbable material. As a furtheralternative, each of the distal anchor and the body comprise anon-absorbable material, connected by an absorbable link. This may beaccomplished by providing a concentric fit between the distal anchor andthe body, with a transverse absorbable pin extending therethrough. Thisembodiment will enable removal of the body following dissipation of thepin, while leaving the distal anchor within the bone.

The components of the present invention may be sterilized by any of thewell known sterilization techniques, depending on the type of material.Suitable sterilization techniques include heat sterilization, radiationsterilization, such as cobalt 60 irradiation or electron beams, ethyleneoxide sterilization, and the like.

The specific dimensions of any of the bone fixation devices of thepresent invention can be readily varied depending upon the intendedapplication, as will be apparent to those of skill in the art in view ofthe disclosure herein. Moreover, although the present invention has beendescribed in terms of certain preferred embodiments, other embodimentsof the invention including variations in dimensions, configuration andmaterials will be apparent to those of skill in the art in view of thedisclosure herein. In addition, all features discussed in connectionwith any one embodiment herein can be readily adapted for use in otherembodiments herein. The use of different terms or reference numerals forsimilar features in different embodiments does not imply differencesother than those which may be expressly set forth. Accordingly, thepresent invention is intended to be described solely by reference to theappended claims, and not limited to the preferred embodiments disclosedherein.

What is claimed is:
 1. A spinal stabilization device for limiting atleast one degree of movement between a superior vertebral body and aninferior vertebral body of a patient, comprising: an elongate body,having a proximal end and a distal end; a distal anchor on the distalend of the elongate body; a retention structure on the body, proximal tothe distal anchor; and a proximal anchor, moveably carried by the body,and including at least one flat surface configured to abut against asurface of an inferior articular process of the superior adjacentvertebral body when the stabilization device is inserted into theinferior adjacent vertebral body, the flat surface extendingsubstantially an entire length of the proximal anchor in adistal-proximal direction; and at least one complementary retentionstructure on the proximal anchor configured for permitting proximalmovement of the body with respect to the proximal anchor but resistingdistal movement of the body with respect the proximal anchor.
 2. Thespinal stabilization device of claim 1, wherein the distal anchorcomprises a helical flange.
 3. The spinal stabilization device of claim1, wherein the retention structure on the body and the at least onecomplementary retention structure on the proximal anchor comprise aseries of ridges and grooves.
 4. The spinal stabilization device ofclaim 3, wherein the at least one complementary retention structure onthe proximal anchor comprises an annular ring positioned within a recessformed between the proximal anchor and the elongate body.
 5. The spinalstabilization device of claim 1, wherein the proximal anchor includes adistally facing surface, the distally facing surface including at leastone bone engagement feature.
 6. The spinal stabilization device of claim1, wherein the proximal anchor has an outer surface, at least a portionof the outer surface being elastic.
 7. A spinal stabilization device forlimiting at least one degree of movement between a superior vertebralbody and an inferior vertebral body of a patient, comprising: anelongate body, having a proximal end and a distal end; a distal anchoron the distal end of the elongate body; a retention structure on thebody, proximal to the distal anchor; and a proximal anchor, moveablycarried by the body and including at least one saddle-shaped surfaceconfigured to abut against an inferior articular process of the superioradjacent vertebral body when the stabilization device is inserted intothe inferior adjacent vertebral body, wherein a centerline of the atleast one saddle-shaped surface extends in a generally distal-proximaldirection; and at least one complementary retention structure on theproximal anchor configured for permitting proximal movement of the bodywith respect to the proximal anchor but resisting distal movement of thebody with respect the proximal anchor.
 8. The spinal stabilizationdevice of claim 7, wherein the proximal anchor has an outer surface, atleast a portion of the outer surface being elastic.
 9. The spinalstabilization device of claim 7, wherein the distal anchor comprises ahelical flange.
 10. The spinal stabilization device of claim 7, whereinthe retention structure on the body and the at least one complementaryretention structure on the proximal anchor comprise a series of ridgesand grooves.
 11. The spinal stabilization device of claim 10, whereinthe at least one complementary retention structure on the proximalanchor comprises an annular ring positioned within a recess formedbetween the proximal anchor and the elongate body.
 12. The spinalstabilization device of claim 7, wherein the proximal anchor includes adistally facing surface, the distally facing surface including at leastone bone engagement feature.